To prepare the brownies: Preheat the oven according to brownie package instructions. Grease two 6” round cake pans with cooking spray and line the bottoms with rounds of parchment paper for easy removal from pan. Alternatively, you can grease one 9” springform pan, also lining it with a parchment paper round.
Prepare brownie batter according to package instructions. Divide the batter evenly between the two 6” pans or single 9” springform pan. Bake the brownies according to package instructions until done. Allow to cool completely. You can expedite this process in the fridge.
To prepare the nuts: preheat the oven to 350 degrees. Combine the chopped nuts, butter, and salt in a small bowl. Spread the nuts out on a small baking sheet and bake in the oven to toast, tossing every few minutes, until slightly darkened and fragrant, about 10-15 minutes. Allow to cool completely.
To prepare the ice cream mixture: In a large bowl or the bowl of a stand mixer, beat the cold cream with a whisk attachment on medium-low speed until foamy. Add ¼ cup of the sugar and increase the speed, whipping until stiff peaks form. Remove the whipped cream to a separate bowl and store in the fridge until ready to use. In that same large bowl or bowl of stand mixer, beat the cream cheese on medium speed until smooth and creamy, about 1-2 minutes. Scrape the sides of the bowl along the way as needed. Add the remaining ¼ cup sugar and vanilla and beat briefly to combine. Fold in ½ of the whipped cream mixture until almost uniform. Add the remaining whipped cream and fold until well combined, being sure to not overwork it. Store this mixture in the fridge until you’re ready to assemble your cake.
To prepare the cake: Remove both brownies rounds from the pan and remove the parchment liner from the bottom of one brownie cake. Line the sides of one of the 6” pans (or the 9” springform pan) with and acetate sheet (see notes) or a sheet of parchment paper barely longer than length of the diameter of the 6” pan, folded in half lengthwise to serve as long, reinforced sheet of parchment. Tape the acetate sheet or parchment to secure along the sides of the pan as needed. If making a 2 layer 6” cake, place the layer of brownie with the parchment sheet still attached into the bottom of the prepared pan. Smooth 1-1/4 cups of the ice cream mixture evenly over top of it. Sprinkle with approximately ¾ of the nuts. Drizzle on 1/3 cup of hot fudge sauce. Place the second layer of brownie on top of the fudge sauce. Smooth an additional 1-1/4 cup of ice cream mixture on top. Place the whole pan in the freezer and freeze until solid, about 6 hours. Place the remaining ice cream mixture in the fridge. If making a single layer 9” cake in a springform pan, place the brownie into the bottom of the lined pan. Drizzle the brownie with ½ cup hot fudge sauce and sprinkle 2/3 of the nuts on top. Spread the ice cream mixture on top, reserving about 1 cup to frost the sides, if desired. Freeze in the freezer until solid, about 6 hours. When ready to frost the cake, carefully remove the cake from the pan and pull off the acetate sheet. Use the remaining ice cream mixture to “frost” the cake. Drizzle the remaining hot fudge sauce on top and decorate the cake with any additional ice cream mixture, nuts, or cherries. Store the cake in the freezer and thaw 5 minutes prior to eating.